Question

1200gm aqueous solution contains 200gm calcium bromide $CaB{r_2}$ .Calculate molality of solution.

Answer 1

Hint : Molality is derived from the term molarity, which refers to a solution's molar concentration. G. N. Lewis and M. Randall appear to have published the first recorded usage of the intense property molality and its adjectival unit, the now-deprecated molal, in their 1923 book Thermodynamics and the Free Energies of Chemical Substances.

Complete Step By Step Answer:
Molality is the number of moles of solute in a solution that corresponds to 1 kilogram or 1000 gram of solvent. The definition of molarity, on the other hand, is based on a certain volume of solution. Molality is measured in mol/kg, which is a standard unit in chemistry. 1 molal is another term for a solution with a concentration of 1 mol/kg. The mol/kg unit requires molar mass to be represented in kg/mol rather than g/mol or kg/kmol. Despite the fact that the two terms are sometimes misunderstood, the molality and molarity of a dilute aqueous solution are almost identical, since one kilogramme of water (solvent) occupies the volume of one litre at room temperature, and a tiny quantity of solute has minimal influence on the volume.
Mass of solvent is nothing but the difference between the mass of the solution and mass of solute.
Hence mass of solvent = 1200 – 200 g
Mass = 1000 g = 1 Kg
Now the mass of solute $CaB{r_2}$ is 200 g
${\text{No of moles = }}\dfrac{{{\text{Given Mass}}}}{{{\text{Molecular Mass}}}}$
${\text{No of moles = }}\dfrac{{{\text{Given Mass}}}}{{{\text{Molecular Mass}}}} = \dfrac{{200}}{{200}} = 1mole$
We know that
$Molality = \dfrac{{No.\,{\text{of moles of solute}}}}{{Mass{\text{ of solvent (in Kg)}}}}$
$Molality = \dfrac{{No.\,{\text{of moles of solute}}}}{{Mass{\text{ of solvent (in Kg)}}}} = \dfrac{1}{1} = 1m$
$\Rightarrow Molality = 1$ .

Note :
Molality is determined by the component chosen to be the "solvent" in a certain combination. If a combination has just one pure liquid material, the decision is evident; however, not all solutions are: in an alcohol–water solution, either one might be considered the solvent; in an alloy or solid solution, there is no apparent option, and all ingredients may be considered equally. In such cases, the preferred compositional definition is mass or mole fraction.